1. Field of the Invention
The present invention relates to organometallic complexes and luminescent solids that emit light by phosphorescence and that are suitable for the light-emitting material, color conversion material, etc. of organic EL elements, lighting apparatuses, etc. The present invention also relates to organic EL elements employing the organometallic complexes or luminescent solids, and to organic EL displays employing the organic EL elements.
2. Description of the Related Art
An organic EL element has a structure in which one or a plurality of thin organic material layers is interposed between a negative electrode and a positive electrode. In the organic EL, a hole and an electron are injected into the organic material layer from the positive electrode and the negative electrode, respectively, the recombination energy, which is generated when the hole and electron are recombined in the organic material layer, causes the emission center of the light-emitting material in the organic material layer excited, and when the light-emitting material falls from an excited state to a ground state, light is emitted. The organic EL element is a light-emitting element which uses this emitted light. The organic EL element has features such as self-luminousness and rapid response, exhibits satisfactory visual properties, is super-slim and lightweight, and is excellent in rapid response and movie display. Thus, it is predicted to be widely utilized for flat panel displays such as a full-color display. Particularly, since a two-layered (multilayered) organic EL element comprising an organic thin film having hole transport property (hole-transporting layer) and an organic thin film having electron transport property (electron-transporting layer) was reported (see, C. W. Tang and S. A. VanSlyke, Applied Physics Letters vol. 51, 913 (1987)), the organic EL elements have been attracting attention as large area light-emitting elements which can emit light at as low voltage as 10 V or less.
In the organic EL element, in order to improve luminous efficiency, a light-emitting layer is proposed that is produced from a fluorescence luminescent host material, as the main material, doped with a small amount of dye having a higher fluorescence luminescence as a guest material and that exhibits high emission efficiency (see, “C. W. Tang, S. A. VanSlyke, and C. H. Chen, Journal of Applied Physics vol. 65, 3610, 1989”).
Moreover, recently, it was shown that it is possible to improve the luminous efficiency of the organic EL element by employing, as a light-emitting material, a phosphorescence luminescent material, which uses emission from the excitation triplet state of molecule, instead of the fluorescence luminescent material, which attracts attention (see, M. A. Baldo, et al., Nature vol. 395, 151 (1998), and M. A. Baldo, et al., Applied Physics Letters vol. 75, 4 (1999)). Light emission from organic material is classified into fluorescence and phosphorescence by the properties of excited state causing emission. In organic EL elements, fluorescence luminescent materials have been utilized due to the reason that general organic material does not give phosphorescence at room temperature. From EL emission mechanism, it is anticipated that the phosphorescent state is generated four times as much as the fluorescent state. Thus, in recent years, attention has been made on the application of a heavy metal complex, which shows phosphorescence at room temperature, to a light-emitting material as a means for achieving high efficiency in EL elements. In case of phosphorescence luminescent material, however, there are few materials which emit strong phosphorescence at room temperature, causing a problem that the material can only be selected from a narrow range of materials.
One of known examples of organic EL elements using an organometallic complex emitting phosphorescence at room temperature is a metal complex comprising a N^N^C type tridentate ligand consisting of two coordinate bonds between a platinum element and nitrogen atom, and one direct bond between the platinum element and a carbon atom, wherein two nitrogen atoms involved in the coordinate bond are arranged adjacent to each other (see, Japanese Patent Application Laid-Open No. 2002-363552).
However, the phosphorescent efficiency at room temperature of this metal complex is not sufficient, and thus there is a problem that the organic EL element using this metal complex has a low light emitting efficiency. On the other hand, it has been reported that platinum complexes comprising a N^C^N type tridentate ligand and a Cl atom which binds to the platinum emit phosphorescence in solution with higher efficiency than the N^N^C type platinum complexes, wherein the N^C^N type tridentate ligand consists of two coordinate bonds between a platinum element and nitrogen atom, and one direct bond between the platinum element and a carbon atom, and the carbon atom is arranged between two nitrogen atoms involved in the coordinate bond (see, J. A. G. Williams et al., Inorganic Chemistry Vol. 42, 8609 (2003)). In case of the organic EL element using this metal complex, however, there was a problem that when a N^C^N type tridentate ligand is synthesized, organic compounds such as an organotin compound, which is difficult to synthesize and handle, must be used as a raw material.
An object of the present invention is to solve conventional problems mentioned above and to achieve the following objects. Specifically, an object of the present invention is to provide an organometallic complex and luminescent solid that emit high-intensity light by phosphorescence and that are suitable for a light-emitting material, color conversion material, etc. of organic EL elements, lighting apparatuses, etc. Another object of the present invention is to provide an organic EL element which uses the organometallic complex or luminescent solid and represents excellent luminous efficiency, etc. A further object of the present invention is to provide an organic EL display which uses the organic EL element, represents high quality, allows a constant average driving current independently of light-emitting pixels, and has satisfactory color balance without changing the light-emitting area; and which is suitable for, e.g. full-color displays.